The Hurd will be considerably more flexible and robust than generic Unix. Wherever possible, Unix kernel features have been moved into unprivileged space. Once there, anyone who desires can develop custom replacements for them. Users will be able to write and use their own file systems, their own `exec' servers, or their own network protocols if they like, all without disturbing other users.

A series of interesting examples is available.

The Linux kernel has now been modified to allow user-level file systems, so there is proof that people will actually use features such as these. It will be much easier to do under the Hurd, however, because the Hurd is almost entirely run in user space and because the various servers are designed for this sort of modification.

Notably, flexibility for the user:

transparent ftp

$ cd /
$ ls

personal filesystem

$ dd < /dev/zero > myspace.img bs=1M count=1024
$ mke2fs myspace.img
$ settrans myspace /hurd/ext2fs myspace.img
$ cd myspace

Just curious, but I keep seeing these (and other similar) concepts being brought up as the amazing selling points of the Hurd, but all of this is entirely doable now in Linux with FUSE or things like it.

Nowadays, at LAST, yes, partly. And only on machines where fuse is enabled. Is it enabled on the servers you have an account on?

I'm not sure if an ftp filesystem has been implemented for FUSE yet, but its definately doable; and loopback filesystems like in your second example have been supported for years.

As a normal user? And establish a tap interface connected through ppp over ssh or whatever you could want to imagine?

What, then, are the major selling points or benefits?

These were just examples, Linux is trying to catch up in ugly ways indeed (yes, have a look at the details of fuse, it's deemed to be inefficient). In the Hurd, it's that way from the ground and there is no limitation like having to be root or ask for root to add magic lines, etc.

It also for instance provides userland drivers, for instance the network drivers are actually Linux drivers running in a separate userland process.

It also for instance provides very fine-grain virtualization support, such as VPN for only one process, etc.

etc. etc. The implications are really very diverse...